1. Field of the Invention
The invention relates generally to disk drive control structures and techniques and in particular relates to improved methods and structures for detecting anomalous conditions within a disk drive. Exemplary anomalous conditions may include, for example: detection of touchdown of the air bearing surface of the read/write head on the surface of the recording medium and detection of quality defects on the surface of the recording medium.
2. Discussion of Related Art
Present day disk drives store and retrieve information on a rotating recording media while a read/write head “flies” in close proximity above the surface of each recordable medium. Typically, multiple such recording surfaces and corresponding read/write heads are integrated within a single disk drive device to allow for enhanced performance and capacity.
The recording media may be either optically encoded or magnetically encoded to store data. In general, an optical recording medium records and retrieves information by altering and then sensing optical properties of the recording medium surface. A magnetically encoded recording surface records and senses magnetic flux changes on the magnetizeable surface of the recording medium. In general, the medium is rotated as the read/write head flies in close proximity above the surface of the recordable medium. Multiple concentric tracks or cylinders are typically configured at incrementing radial positions on the recording medium surface and each track consists of a plurality of bits representing the stored data and associated meta-data.
During normal operation of a disk drive, it is critical that the read/write head should not make contact with the surface of the recording medium for an extended period of time. Such physical contact between the read/write head and the surface of the recording medium may cause permanent damage to the read/write head and/or to the recording medium surface thereby potentially losing previously recorded data and/or disabling the disk drive. To this end, an air bearing surface (“ABS”) is designed to maintain a relatively constant “flying height” of the read/write head over the recording medium surface. The mechanical design of the air bearing surface of the read/write head is the principle control mechanism to maintain a safe flying height above the recording medium surface rotating adjacent the read/write head. Since the recordable medium rotates at a relatively constant speed, and since other environmental parameters are relatively stable, the aerodynamic properties of the air bearing surface maintain a substantially constant spacing between the read/write head and the rotating recording medium.
Various control mechanisms are applied to refine control of the flying height of the read/write head above the rotating recording medium. During the manufacture process of a disk drive, it may be desirable to permit touchdown or contact between the read/write head and its corresponding recording medium surface so as to calibrate the feedback control mechanisms used to more accurately maintain a constant flying height. By allowing such contact or touchdown of the read/write head on the recording medium, the feedback control mechanisms for maintaining head flying height may be properly calibrated for a baseline value associated with touchdown or contact with the recording medium surface. Thus, slight variations in mechanical tolerances and electrical properties of the disk drive as well as those of the recording medium surface may be accounted for by calibrating each head for touchdown with its corresponding surface.
Present techniques for determining or detecting such head-surface contact or touchdown during manufacturing test and calibration procedures generally utilize other sensing mechanisms within the disk drive that are indirectly indicative of head touchdown. For example, most presently known touchdown or contact detection mechanisms utilize the position error sensing (“PES”) control features of the disk drive to sense the likely occurrence of touchdown between the read/write head and its corresponding recording surface. The PES sensing and control mechanisms of the disk drive perform fine adjustments to properly position the read/write head radially to a desired track or cylinder location. The PES feedback mechanisms senses errors in attempts to read servo information recorded at fixed locations on the rotating recording medium. As the fly height of the read/write head changes, within a few rotations of the recording medium, the position error sensing feedback control mechanism may determine that a significant enough change or trend of changes in the position error signals may represent a head touchdown.
A number of problems arise in existing touchdown detection mechanisms that depend on sensing changes in the PES related signals. First, detecting head touchdown from such a PES feedback loop signal may require multiple rotations of the rotating recording medium due to the relatively slow control feedback loops used in such PES control systems. Permitting multiple revolutions of the rotating recording medium when a head touchdown condition is presently occurring may cause undue wear of the recording surface and/or of the read/write head. In addition, the PES feedback control mechanisms are significantly impacted by the skew angle of the read/write head relative to the present radial track position. The skew angle is the angle of the read/write head relative to the tangential angle of a given track over which the head is positioned. Thus, use of the PES feedback in control mechanisms for the additional purpose of detecting touchdown of the read/write head on its corresponding recording medium surface may be limited to certain specific ranges of radial positions where the skew angle is large. At track locations where the skew angle is small or zero, this approach will have difficulties because there is no tangential force.
It is evident from the above discussion that a need exists for an improved method and structure for detecting touchdown of the read/write head on its corresponding recording medium surface.